Many electronic control Units (ECUs) are mounted in a vehicle and connected to a communication network. The ECUs perform communication with each other to obtain necessary information, and perform control of respective actuators. As the communication between those ECUs, an in-vehicle local area network (LAN) is used. In a portion requiring high speed communication (˜1 Mbps) in a control system of a vehicle, a controller area network (CAN) is often used as a communication protocol. In a body system portion not requiring high speed communication, a local interconnect network (LIN) is becoming a standard.
With respect to circuit elements forming the ECU for communication, the following problem arises. In a state where many ECUs are connected to a bus for communication, in one or some of them, there can occur such a failure as ground disconnection. This disconnection is that the connection between a circuit ground and a ground as a potential reference is shut-off. In this case, the circuit ground of the ECU, which is brought into an open state by the GND disconnection, may rise to the same potential as the power source voltage.
When such a state occurs, current may flow in reverse from the circuit ground of the ECU to the bus for communication. In the ECU for communication and in a receiver circuit as a part to receive a signal transmitted through the bus, a comparator is used to compare a voltage level of a received input signal with a reference voltage level to convert it into two values of high and low. The comparator is constructed by using a differential amplifier circuit.
When the “GND disconnection” occurs, the current flows in reverse from the circuit ground side of the comparator to the input terminal side. Thus, it becomes impossible to perform a normal comparison operation, and in the worst case, the communication of the network is likely to be blocked.